N-type semiconductive metal sulfide thin films are used as buffer layers between the window layer and the absorber layer in solar cells, thereby allowing a significant increase in efficiency. A “buffer layer” is understood to be a layer having a higher band gap than the adjacent semiconducting absorber layer. This higher band gap can be achieved by alloying or by suitable material selection. Recombination in the interface area of the pn junction is reduced by improved interface conditions and improved band-gap adjustment, leading to an increase in the open terminal voltage. CdS is an n-type semiconductor having a band gap of 2.4 eV and thus absorbs in the UV and visible regions of the solar spectrum. The thereby generated electron-hole pairs are not separated by the space charge region and, therefore, do not contribute to the current. The function of the buffer layer is varied and has not yet been unequivocally clarified in all details. This layer protects the absorber layer from damage and chemical reaction during the deposition of the transparent contact layer; e.g., a ZnO layer.
Increasingly, efforts are being made to substitute the toxic CdS layer by less toxic materials. In this connection, various variants of the CVD method are increasingly used, which also allows for good coverage of rough substrates. In particular, indium(III) sulfide (In2S3), which has an indirect band gap of 2 eV to 2.2 eV, is a promising candidate to replace the toxic CdS. Therefore, various methods for producing it have been described in the art. However, to date, CVD methods are not used for producing it because the materials to be used tend to form solid reaction products already in the gas phase, resulting in inhomogeneous films with poor coverage on the substrate.
Mainly two sequential methods are used for producing indium sulfide thin films. These are the ion layer gas reaction (ILGAR) method and the atomic layer chemical vapor deposition (ALCVD) method, also known as atomic layer epitaxy (ALE) and as atomic layer deposition (ALD). In the ALCVD method, individual layers of atoms of a precursor material are deposited (adsorbed) on a substrate and then caused to react with the second precursor. In the process, ordered stacked (epitaxial) layers of molecules are formed, which allow the production of extremely thin (“monomolecular”) indium sulfide films. Through cyclic repetition, it is possible to deposit a plurality atomic layers one upon another. Due to this procedure, the process is inherently very slow and hardly suitable for producing films having thicknesses of several 10 nm on an industrial scale. Moreover, unreacted atoms of the first precursor in lower layers cannot be caused to undergo reaction later, and may therefore result in defects. In contrast, the ILGAR method does allow complete and thorough reaction of disordered layers of the first precursor material. In the ILGAR method, the first precursor material is completely dissolved in a solvent to form a homogeneous liquid phase, and is applied to the substrate by spraying or dipping, so that the ions of the first precursor material are uniformly adsorbed on the substrate in a disordered form. In the case of dipping, the solvent or residues thereof are removed by drying or evaporation, so that the thin, solid precursor layer can easily be accessed by the gaseous second precursor, which is added in the next step. In this manner, an efficient, thorough reaction of the entire film is achieved, which accurately follows the surface morphology of the substrate, so that even porous substrates can be efficiently covered. The film thickness can be adjusted by cyclically repeating these method steps, it being possible to influence the optical film properties in the process. However, in prior art film production methods, special care must be taken to prevent contact of the two precursors during the process, because otherwise premature reaction (i.e., powder formation) may occur in the gas phase, which would result in poor film adhesion and homogeneity, and thus in films of inferior quality. For this reason, thorough purging with an inert gas is carried out between the sequential steps of the method.
The use of ALCVD for producing In2S3 layers is described, for example, in T. Asikainen et al.: “Growth of In2S3 Thin Films by Atomic Layer Epitaxy” Appl. Surface Science 82/83 (1994) 122-125. This method uses H2S gas and InCl3 which is evaporated at 275° C. from the solid phase and adsorbed on a substrate surface heated to 300° C. to 400° C. In N. Naghavi et al: “High Efficiency Copper Indium Gallium Diselenide (CIGS) Solar Cells with Indium Sulfide Buffer Layers Deposited by Atomic Layer Chemical Vapor Deposition (ALCVD)” Prog. Photovolt: Res. Appl. 2003; 11:437-443, an ALCVD method for buffer layers of solar cells is described, in which indium acetylacetonate In(acac)3 evaporated at 125° C. and H2S gas are used at substrate temperatures of 160° C. to 260° C. However, it is generally understood by those skilled in the art that such a method is far too slow and too expensive to be used for producing solar cells on an industrial scale.
WO 93/04212 A1 describes that preferably well-oriented crystalline thin films can be made from compounds of group III and group VI elements using a carrier-gas based, single source CVD process. However, in this method, temperatures between 350° C. and 650° C. are required to crack the organometallic compounds which are used as precursors and which first have to be prepared from metal halides or the like, which is complicated and expensive. Hydrogen sulfide may indeed be added to the carrier gas during the preparation of indium sulfide films; but no information is given on the amounts in which to add hydrogen sulfide. Rather, in the exemplary embodiments in which only argon carrier gas is used, no mention is made of adding hydrogen sulfide. U.S. Pat. No. 5,112,650 describes that in chemical vapor deposition of metal chalcogenide thin films, addition of hydrogen sulfide to the carrier gas is theoretically possible, but in practice not suitable in the case of insoluble metal sulfides, because it results in unwanted powder formation and, thus, inhomogeneous film formation.
The ILGAR method operates in an energy-efficient manner at ambient/atmospheric pressure and at a temperature which is dependent on the ion-exchange behavior of the materials used (typically ambient/room) temperature). This method is described in principle in DE 198 31 214 C2 (modification: sulfide formation by ILGAR via a hydroxide intermediate, DE 199 16 403 C1). The contents of the aforementioned documents are considered part of (and are incorporated by reference in) the disclosure of the present invention.
N. A. Allsop et al.: “Spray-ILGAR Indium Sulfide Buffers for Cu(In, Ga)(S,Se)2 Solar Cells” Prog. Photovolt: Res. Appl. 2005; 13:607-616 (“ALLSOP 2005”) and N. A. Allsop et al.: “Indium Sulfide Thin Films Deposited by the Spray Ion Layer Gas Reaction Technique”, Thin Solid Films 513 (2006) 52-56 (“ALLSOP 2006”) describe an ILGAR method including spray deposition of the dissolved precursor (spray ILGAR) for producing In2S3 buffer layers for use in chalcopyrite solar cells. In the ILGAR method generally described above, which includes strictly separate process steps (sequential method), indium chloride (InCl3) is initially dissolved in ethanol and then atomized to form a spray (aerosol=mixture of liquid particles and air) using an ultrasonic generator. This spray is transported to the heated substrate with the aid of an inert nitrogen gas stream (N2). To allow a good deposition rate, the substrate is at a temperature between 300° C. and 450° C. (see ALLSOP 2006, FIG. 2). In principle, however, efficient film formation occurs already at temperatures as low as 100° C. Especially when growing a buffer layer on an absorber layer, care must be taken that the absorber layer is not damaged and that the heating temperature is controlled at a level clearly below 300° C., typically between 175 and 250° C. The indium ions deposited on the substrate surface are then treated with hydrogen sulfide gas, so that the desired reaction to form In2S3 takes place on the substrate surface. In addition, purging with N2 is carried out between each of the individual steps of the method to prevent contact of the two precursors in the gas phase.
The basic design of a system suitable for carrying out the above-described method can be seen from FIG. 1 of ALLSOP 2006. The valve shown there in the hydrogen sulfide supply line is a shut-off valve. This valve is opened only in the second sequential step of the method. This is when the first sequential step of the method is complete and the ultrasonic generator is off, so that no spray (aerosol) is produced anymore. In addition, purging with N2 has been completed at this point, so that there is no material of the first precursor in the reaction vessel anymore. Thus, this shut-off valve serves to ensure a strictly sequential process and knows only the “open” and “closed” positions.